Polyamide resins are excellent in strength, heat resistance, and chemical resistance and is excellent in specific gravity, i.e., has a smaller specific gravity than that of metals. Therefore, the polyamide resins have heretofore been used as alternative materials for metals in automobile mechanical parts, etc.
Particularly, members around engines are required to have durability in a high-temperature environment. Therefore, various polyamide resin compositions excellent in heat aging resistance have been proposed (see e.g., Patent Literatures 1 and 2).
In the present specification, the “heat aging resistance” refers to resistance to so-called thermal oxidation, by which a molded product can maintain practically sufficient mechanical characteristics and has only a little color change when left for a long time in an air atmosphere under high-temperature conditions equal to or lower than the melting point with the shape of the molded product maintained.
In recent years, the downsizing of automobiles has been practiced as one approach for improvement in fuel efficiency. As a result, automobile engine room parts tend to have a higher density, and engine rooms tend to have a higher internal ambient temperature.
In addition, higher powers have been imparted to engines with superchargers for improvement in fuel efficiency. Along with this, engine rooms tend to have an increasingly higher ambient temperature.
Thus, there has been a demand for polyamide resins having heat aging resistance over a long period under higher-temperature conditions than ever. Specifically, there has been a growing demand for durable materials that can maintain practically sufficient mechanical characteristics and have only a little color change even when used for a long time in an air atmosphere under high-temperature conditions of 150° C. to 230° C.
A technique which involves adding a copper compound (an oxide or a salt of copper) is known as a technique for improving the heat aging resistance of polyamide resins.
Likewise, a technique which involves mixing a copper compound and iron oxide with 2 types of polyamides differing in melting point (see e.g., Patent Literature 3), a technique which involves mixing fine particles of element iron with polyamide (see e.g., Patent Literature 4), and a technique which involves mixing a fine dispersed metal powder with polyamide (see e.g., Patent Literature 5) are disclosed as techniques for improving the heat aging resistance.
Meanwhile, a polyamide resin composition containing sodium aluminate and a method for producing the same are disclosed (see e.g., Patent Literatures 6 to 12).
The polyamide resin composition containing sodium aluminate has heretofore been known to have excellent heat retention stability.
The “heat retention stability” refers to characteristics by which the resin is less decomposed and deteriorated when the polyamide resin composition is kept at a temperature equal to or higher than the melting point and is thereby in a melted state, and consequently, reduction in mechanical physical properties or color change of the polyamide resin composition caused by the action of keeping it at the temperature equal to or higher than the melting point is prevented.
Also, a technique which involves adding a resin having a lower melting point and a heat stabilizer to a polyamide resin is disclosed (see e.g., Patent Literature 13).